[sdiy] VCO and expo converter question

[Scott Bernardi]

I wouldn't worry too much about the loading effects on the coarse tune 
pots; it just means you'll get a bit more change per % rotation near the 
end of the rotation than at the beginning. We usually tune by ear, so 
the fact that it's not perfectly "calibrated" doesn't matter too much. A 
lot of people do use 100K pots with 100K loads.
Harry says exponential FM is more common too. It's funny - most of all 
the circuit schematics I've seen had a linear FM input, and that's how 
I've built all mine. Exponential FM is just putting an attenuator on a 
1v/octave input, which you already had.
I'll have to play around and compare the differences in sound between 
the linear and exponential FM now.


ryan wrote:

> nice tutorial. thanks!
> I'm sure glad I asked here before getting the PCB made. I thought that 
> Exponential FM would have been more typical, thats why I didn't add 
> Linear FM. Yesterday was actually the first time I tried putting 
> something into the FM jacks on my synth.  well, Before yesterday, I 
> had only 1 VCO. the Idea for this VCO is to be a simple, fit on a 
> small PCB, and take up little panel space. I have Oakley Sound VCOs 
> for my main oscillators, but I'd like to get this one to stay in tune 
> with those.
>  
> for the pots. I'm using 100K pots on a 100K load. arg. I already spent 
> a bunch of $ on those fancy spectrol 100k pots. I think it'd be easier 
> to just double the values of all the other resistors. I think that 
> would atleast reduce the warping enough and the 10K trimmer for V/Oct 
> would be more centered around what the value should actually be. I 
> looked at that java applet, It looks pretty nasty the way I have the 
> circuit now.
>  
> about the coarse adjustment, I suppose +/-5 octaves would be alot more 
> useful. meant to change that.
>  
> thanks !
> ryan
>
>     I've got a tutorial on pots at
>     http://home.comcast.net/~sbernardi/elec/og2/partsub_pots.html.
>     In general to avoid loading effects, you want your voltage divider
>     pots to be 5 to 10 times less than the resistance that loads them.
>     So for a 100K input resistor, use a 10K up to 50K pot. To see the
>     effect of loading, I have a link to Chris List's java applet that
>     plots loading effects.
>     You might want to decrease R2 a bit to give yourself more Coarse
>     freq adj range. As you have it, you'll only get 6 octaves. Using a
>     300K will give you a 10 octave tuning range.
>     The FM input you have is feeding the exponential input - linear FM
>     is more typical. To get linear FM, move the end of R28 to pin 6 of
>     U7B. Also, you'll want to change the value of  R28 to something
>     above 1M.
>     I would also have a second 1v/octave input - duplicate the R27
>     input.  I also find having a front panel "LFO switch" useful. This
>     would sum a large negative voltage into your input summer, which
>     would switch the frequency way down. -15v through 300K into the
>     summing amp with switch you down 5 octaves.
>
>     The 100pF compensates for the extra phase shift running  IC1A in
>     the feedback loop of the opamp, and it is to prevent oscillation.
>     I also use 100pF because everybody else does.
>

-- 
Scott Bernardi
sbernardi at comcast.net

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